To provide appropriate humidity is an important factor for providing a comfortable indoor air atmosphere. When the humidity is deficient, there may be caused adverse influences such as human health hazard, deterioration of objects, and generation of static electricity. To provide appropriate humidity, for example, in the Building Sanitation Control Act, it is determined that, in specific buildings such as commercial facilities and offices having floor areas of 3,000 m2 or more, the temperature is required to be maintained at 17 degrees Celsius to 28 degrees Celsius, and the relative humidity against the temperature is required to be maintained at 40% to 70% as control standard values for the air environment. Further, in American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), it is clearly specified that the relative humidity is from 30% to 60% as a humidity criterion.
As an indoor space humidification method of humidifying an indoor space, there has been known an evaporative method. The evaporative method is a method of performing humidification by preparing a water absorbing humidifying material having water absorption capability, supplying water to the water absorbing humidifying material, and causing air to pass through the water absorbing humidifying material. When the air is caused to pass through the water absorbing humidifying material, the water contained in the water absorbing humidifying material is subjected to heat exchange with an air current, to thereby cause vaporization and evaporation. In this manner, the indoor space is humidified (for example, Patent Literature 1 and Patent Literature 2).